Testing Cable Capacitance

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corwin99

Testing Cable Capacitance
« on: 20 Dec 2004, 06:32 pm »
How does one go about testing cable capacitance? I have read a couple articles on this and have found nothing conclusive. I've been told that to check it i should use my metre to connect to the ground and signal of my cable and connect a capacitor to the other end of the cable's signal and ground and compare that number to the number of the cap tested without the cable and that would be my capacitance, however i have found no definitive answer.

Can Anyone help? :D

Occam

Testing Cable Capacitance
« Reply #1 on: 21 Dec 2004, 04:29 am »
I'm assuming you're asking how to measure the paralell capacitance of interconnects and speaker cables? For this you'd generally need a specific capacitance or LCR meter. The capacitance measurement capabilities of various digital multimeters, like Fluke, generally don't have the ideal  resolution in your range of interest, 10-1,000pf. If you need a generalized measurement instrument, you might find the limited resolution adequate given other potential uses.

The next step up is a specific capacitance meter with a sufficient low range scale of 200pf, but pretty much only measures capacitance. A meter such as the BK 810c , $70,  does this well, albeit at fixed frequencies.

The next step would be a LCR Meter, which  often measure additional parameters, capacitance,  inductance, Resistance, and depending on model, Q, various semiconductor parameters, as well as , capacitive dissapation factors... at differing frequencies. If you're serious about charaterizing loudspeaker cables, you really want the ability to measure inductance. Its an added bonus if you wind you own coils for loudspeaker crossovers... A reasonable (not top of the line industrial) would be the BK 875b.
http://www.tequipment.net/TMT.html

[and some really old fogeys will remember when you could use an 'dip' meter to measure almost anything....]

corwin99

Testing Cable Capacitance
« Reply #2 on: 21 Dec 2004, 04:38 am »
I already have a Capacitance meter (kind of, its a multimeter that measures capacitance but only down to 100pF at the last decimal place)... I was just wondering how to measure it.. do i just hook up the signal to one terminal and the ground to the other and it is that value? Most of the cables are giving me readings of about 200pF give or take over a 3ft length.

I think i will want something with better resolution though.. something that gives me a better amount.. maybe one more decimal place over.. so i can tell if that is 249pF or 200pF...

But am i measuring it the right way?

Occam

Testing Cable Capacitance
« Reply #3 on: 21 Dec 2004, 04:55 am »
For example -

Speaker cables -
Hook up the amp end leads to the black and red leads of your multimeter. Leave the speaker ends unconnected and not touching each other. This will give you the total paralell capacitance, including bi/tri wire runs.

Interconnects, same but connect the red to the center (signal pin) conductor. and the black to the outer ground connection at the source end. Leave unterminated on the load end.

corwin99

Testing Cable Capacitance
« Reply #4 on: 21 Dec 2004, 05:06 am »
Awesome. Thank you so very much, Occam.
 :mrgreen:  :D